Merge branch 'ma/more-opaque-lock-file' into maint

Code clean-up. * ma/more-opaque-lock-file: read-cache: try not to peek into `struct {lock_,temp}file` refs/files-backend: don't peek into `struct lock_file` midx: don't peek into `struct lock_file` commit-graph: don't peek into `struct lock_file` builtin/gc: don't peek into `struct lock_file`